The invention discloses a two-dimensional photonic crystal-based multi-gas sensing system. The system comprises a tunable optical generator, a two-dimensional photonic crystal and an optical power detector. Three different resonant cavities are arranged on the two-dimensional photonic crystal. Two parallel photonic crystal waveguides are arranged below and above the resonant cavities. Light wave produced through the tunable optical generator is guided into the inlet waveguide of the two-dimensional photonic crystal through optical fibers, then is coupled and enters the resonant cavities, and the light with resonant wavelength is coupled and enters the outlet waveguide of the two-dimensional photonic crystal and then is derived from the two-dimensional photonic crystal through an optical fiber. The optical power detector is located at a light wave outlet optical fiber end. The detection method comprises 1, introducing light into the photonic crystal waveguide, 2, coupling the input light wave and feeding the light wave into the resonant cavities, 3, producing resonance absorption under action of resonant wave, the outlet waveguide and gas to be detected, and 4, measuring the output light power. The sensing unit is simple and has a small volume, stable and reliable performances and a high detection precision. The sensing system can realize synchronous detection of concentrations of multiple types of gas.